NASA’s New Horizons spacecraft is expected to pass close to a tiny rock on the Kuiper belt on 1 January.

The unmanned vessel will pass the cosmic object known as "Ultima Thule", at a distance of just 3,500km.

It is the most distant object ever studied at some 6.4 billion kilometres away.

Ultima Thule is about the size of the US capital, Washington, and orbits in the dark and frigid Kuiper Belt about a billion miles beyond the dwarf planet, Pluto.

The spacecraft's closest approach to this primitive space rock comes on 1 January at 5.33am GMT.

Until then, what it looks like, and what it is made of, remain a mystery.

"This is a time capsule that is going to take us back four-and a-half billion years to the birth of the solar system," said Alan Stern, the principal investigator on the project at the Southwest Research Institute.

A camera on board the New Horizons spacecraft is currently zooming in on Ultima Thule, so scientists can get a better sense of its shape and configuration - whether it is one object or several.

"We've never been to a type of object like this before," said Kelsi Singer, New Horizons co-investigator at the Southwest Research Institute.

About a day prior, "we will start to see what the actual shape of the object is," she said.

The spacecraft entered "encounter mode" on 26 December, and is "very healthy," added Stern.

Communicating with a spacecraft that is so far away takes six hours and eight minutes each way - or about 12 hours and 15 minutes round trip.

New Horizons' eagerly awaited "phone home" command, indicating if it survived the close pass is expected 1 January at 3.29pm GMT.

Until then, the New Horizons spacecraft continues speeding through space at 51,500 kph, travelling almost a million miles per day.

NASA scientists are eagerly awaiting the first images.

"Because this is a flyby mission, we only have one chance to get it right," said Alice Bowman, missions operations manager for New Horizons.

The spacecraft, which launched in 2006, captured stunning images of Pluto when it flew by the dwarf planet in 2015.